The present invention generally relates to welding processes and materials. More particularly, this invention relates to a process for welding alloys that are prone to cracking when welded.
Components of gas turbine engines, such as blades (buckets), vanes (nozzles) and combustors, are typically formed of nickel, cobalt or iron-base superalloys with desirable mechanical properties for turbine operating temperatures and conditions. Notable examples are gamma prime precipitation-strengthened nickel-base superalloys, particular examples of which include René 125, René 80, René N5, René N4, René 108, GTD-111®, GTD-444®, IN738, IN792, MAR-M200, MAR-M247, CMSX-3, CMSX-4, PWA1480, PWA1483, and PWA1484. Each of these alloys has a relatively high gamma prime (principally Ni3(Al,Ti) content as a result of containing significant amounts of aluminum and/or titanium. As the material requirements for gas turbine components have increased with higher operating temperatures, various processing methods have been used to enhance the mechanical, physical and environmental properties of components formed from superalloys.
During the operation of a gas turbine engine, turbine components are subjected to various types of damage or deterioration, including wear and cracks. Because the cost of components formed from superalloys is relatively high, it is more desirable to repair these components than to replace them. For the same reason, new-make components that require repair due to manufacturing flaws are also preferably repaired instead of being scrapped. However, gamma prime precipitation-strengthened nickel-base superalloys have poor fusion weldability due to their liquidation cracking and, especially, strain-age cracking (SAC) tendencies.
Typical ductility characteristics of gamma prime precipitation-strengthened nickel-base superalloys are represented in FIG. 1 relative to temperature. Liquidation cracking occurs within a brittleness temperature range (BTR), usually between the solidus temperature (Ts) and the liquidus temperature (T1) of the material, and strain-age cracking occurs in a ductility drop temperature range (DTR), usually between Ts and about 0.5Ts, identified in FIG. 1. Strain-age cracking is a significant problem that exists in many nickel-based alloys, and is manifested by the drop in ductility within the ductility drop temperature range of an alloy during cooling. As a result of this phenomenon, susceptible materials often exhibit cracking in the weld metal heat affected zones (HAZ), including the area of the fusion zone re-heated during multiple pass welding, which are exposed to the ductility drop temperature range during the weld thermal cycle. Although the problem is fairly common, the underlying mechanism of ductility drop is still not fully understood.
Various methods have been used to avoid the strain-age cracking problem. These methods include preheating the alloys prior to welding to limit thermal stresses, using a low heat input source for welding, slowly cooling after welding to limit thermal stresses within the alloy, and over aging the alloy thereby reducing its creep resistance and allowing stress relaxation to take place more readily. Interpass temperature control is a commonly used welding method that requires an operator to pause between weld passes with a welding device to allow the weld alloy/weldment temperature to drop to a desired temperature, typically less than 350° F. (177° C.), before the next weld pass. This welding method is used to prevent deterioration of the weld metal and heat affected zone properties, especially when notch toughness is an important factor. However, due to poor inherent weldability of certain superalloys, such as precipitation-strengthened nickel-base superalloys, such welding techniques may have a limited affect in controlling the size and quantity of cracks.
U.S. Pat. No. 6,333,484 discloses a process for welding a nickel or cobalt based superalloy article to minimize cracking by preheating the entire weld area to a maximum ductility temperature range, maintaining such temperature during welding and solidification of the weld, raising the temperature for stress relief of the superalloy, and then cooling at a rate effective to minimize gamma prime precipitation. A disadvantage of this process is its reliance on an external induction heating coil as a heating source. Further, the temperature of the weld area must be controlled throughout the process. In view of this, improved methods are desirable for welding precipitation-strengthened superalloys, and particularly gamma prime precipitation-strengthened nickel-base alloys, by which strain-age cracking can be reduced or avoided in weldments.